Local area networks (LANs) typically provide for communication between computer terminals located in the same room, building or complex of buildings by electronic connection between the terminals with conductive wires or optical fibers. Wireless communication between terminals using infrared (IR) light emitting diodes (LEDs) is known but suffers from several drawbacks. One drawback of IR LEDs is a lack of connectivity at distances over perhaps fifty feet that is caused by reduction of signal intensity to power levels inadequate to distinguish over ambient IR noise. Another drawback is that of crosstalk between terminals, so that information transmitted from a terminal is not clearly received by another terminal.
Some prior art systems for infrared communication use mirrors to deflect the light between stations. In U.S. Pat. No. 4,017,146, Lichtman teaches angular distribution of a concentrated laser beam by deflecting the beam with a moving mirror so that the beam raster scans over a large angle. The beam impinges upon a given spatial location in short pulses that amount to a small fraction of a given time period, the remainder of the time period being essentially devoid of the beam at that location, and thus the transmission of data being similarly limited. U.S. Pat. No. 4,982,445 to Grant et al. teaches a laser beam communication system for spacecraft utilizing mirrors positioned in the path of the beams which adjust reflection to different angles.
Other systems employ high and low data transmission channels. U.S. Pat. No. 5,321,542 to Frietas et al. teaches an optical communications system utilizing a high bandwidth, high speed infrared data channel along with a more robust, low bandwidth, low speed infrared channel for maintaining communication when the high speed channel is obstructed. U.S. Pat. No. 5,229,593 to Cato teaches a free space laser communication system operating at a high power level for optimum data transmission when a path between terminals is not blocked, and operating at a lower, eye-safe power level when the path between terminals is obstructed.
Still other systems have characteristics that depend upon the medium of transmission. U.S. Pat. No. 5,227,908 to Henmi teaches an intensity modulated infrared signal for improved noise reduction transmission via an optical fiber. U.S. Pat. No. 5,181,135 to Keeler uses light sources tuned for minimizing losses in an underwater communications system. U.S. Pat. No. 5,159,480 to Gordon et al. teaches a communication system for naval vessels that sends out a horizontally dispersed, vertically concentrated infrared signal for receipt by a remote receiver.
Despite these advances in free space communication, certain obstacles remain. Some known systems utilize a form of time multiplexing to avoid confusion between signals from different terminals, thus cutting into the time available for data transfer between separate terminals. Similarly, the frequency with which infrared diodes can be modulated also can limit the speed with which data can be transferred. Moreover, detection of the signals is often thwarted by ambient infrared noise. Furthermore, a free space local area network including terminals disposed in separate rooms has difficulties caused by walls separating the rooms. The term "free space" is meant to signify that a path through air is available between terminals, although the path need not necessarily be direct. For the situation in which walls substantially seal one room from another, a free space path is not present.
It is an object of the present invention to overcome the aforementioned obstacles.